Electric measuring instrument



AprilV 8, 1930. H T, FAUS ELECTRIC MEASURING INSTRUMENT Filed Sept. 27, 1928 Inventor-z I-larold T. FEMS; bym/,97mm

I-iLs Attorney.

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ll/.lyinvention relates to electric measuringinstruments and its main object is to provide a rugged low cost miniature instrument capable ot giving good accuracy with a minimum f operating current. .ein example of one application where such an instrument is needed is in the use oit voltmeters for radio receiving sets. ln order to measure the tube voltages accurately it is essential'that the voltmeter take a very small operating current since otherwise the current taken by the voltineter materially reduces the voltage which it is desired to measure, particularly where batteries and B battery eliminators constitute the source. i

Some of the higher grade Dhirsonval instruments might be used, but the cost ot such instruments is too great to find extensive application on radio receiving sets. 'lhe instrument oil my invention., while capable ot giving very good measurement accuracy at eX- tremely low operating currents, is less expensive and more rugged than the usual instrument which itis intended to supersede.

lin carrying my invention into eidect l provide a measuring instrument with a permanent magnet lield having a narrow air gap in which a substantially iiat moving coil is inserted. By providing a narrow air gap a high density held is provided so that ample torque on the moving coil is produced with small eX- isting currents. The current to the moving coil is preterably conveyed through the bearings where the instrument is intended itor use as a voltmeter with small operating currents so that the usual `spiral spring connections may be dispensed with. llnstead ot the usual return spring l pre-ler to employ a magnetic member on the shalt arranged to operate in the permanent magnetic tield to furnish an auxiliary zerorestoring torque tor the instrument.

'lhe -Features et my invention which are s believedto be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of the invention, reference is made in the followingdescription to 'the accompanying drawings in which Fig. 1 is a perspective view of the preferred form ot the instrument; Fig. 2 isa side view oi' the ln Fig. 1 l have represented a `voltmeter constructed Vin accordance with the present invention. vlhe instrument comprises a permanent magnet 10 having an air gap 11. 'er

magnetic pole piece 12 is provided adjacent u the air gap which is connected to one pole of the permanent magnet and overhangs the adjacent pole piece so as to provide a narrow air gap across which a relatively high density lux exists. lhe dat movin coil 13 oi the instrument is inserted between t e interleaved pole piece members in this narrow air gap. This coil is a pancake coil formed by winding a relatively large number oit turns oit une insulated wire on a dat spool having side pieces 14 and 15 made preferably oit aluminum and riveted together as indicated at 16 Fig. 2. The inner and outer ends ot this coil are connected to the upper and lower pivots 1'? and 18 as shown in Fig. 2. IFliese pivots are metallic and are insulated it'rom each other by being secured in a rod 19 otinsulating matey porting the end ot pole piece 12 and are made ot non-magnetic material. i For the small operating current for voltmeters `the upper andjlovver bearings may comprise metal cups 21 made oit some such material as phosphor bronze. represented, the upper bearing is held in place by a spring leali 22 secured to the pole piece 12. The lower bearing may be rigidly secured to the base plate ot the instrument. The spring leaf 22 is adjusted to hold the moving element lightly but firmly in rotative position with the bearingpivots in the depressions ot the cup-shaped bearings. l high resistance spool not shown., but preferably mounted in the back of the case is connected in series with the lil@ moving coil which is connected through the bearings to the terminals 24 and 25, as represented in Fig. 1. These terminals extend to the back of the case for the circuit connections. For av voltmeter for use with radio sets I have found the following specifications give satisfactory results. The coil 13`may have about 1800 turns and a resistance of about 600 ohms; the resistance in series may be a 50.000 ohm nichrome resistance element. This gives about 1000 ohms per volt drop in the instrument'circuit or a current of about nlm, ampcres through the instrument on a 50 volt circuit. In spite of this small 'current through the moving coil, ample torque is produced by reason of the rather intense magnetic field in which the coil is placed. The coil may be connected.tomoveintoor out of the permanent magnetic fieldtoproduce a measuring deflection.` As represented in Fig.v 1. it is arranged to be repulsed or to move out of the field when voltage is applied; that is to say, the polarity of the field produced by coil 13 is such as to repel that produced by the permanent magnet so that thesefields oppose each otherand the coil tends to move sideways to the left out of the narrow air gap. If desired. the instrument may be arranged for both repulsion and attraction and have a'zero center scale so that when the current is in one direction through the coil it moves in' one direction and when the current is reversed it moves in the opposite direction.

In order that the deflection of the instrument shall be proportional to the current in'coil 13 it is necessary to provide a counter torque. This is preferably accomplished by means of an oblong magnetic vane 26 on the` shaft located between the poles 32 and 33 of a magnetic diverter represented 1n Fig. 4. It will be noted that this vane is mounted well outside the influence of the field pro- I duced by the armature coil. The source of magnetism for this diverter is the permanent magnet 10 and a small amount of its flux is diverted through magnetic members 34 and 35 which extend beneath the pole tips of the main magnet. Thel outer ends of these members may be seen in Fig. 1. The

-pole pieces 32 and 33 are mounted on a nonmagnetic plate 36 which may be rotated about the pivot point 21 for the -purpose of adjusting the .zero setting of the pointer. The amount of flux diverted through the zero restoring vane 26 mounted on the shaft may be accurately adjusted to the correct amount by adjustment of one or more of the air gaps in the path of thc diverted flux.

'That part of the flux of the permanent magnet passing through the magnetic vane tends to move the vane to the position of least reluctance which corresponds to the zero position of the pointer 20. The vane is shaped, and proportioned to produce a counter torque substantially proportional to the instrument'torque whereby a deflection proportional to the current flowing through coil 13 is produced. This magnetic vane 1s preferably made of a material which has high permeability and low hysteresis.` The material known as permalloy consisting of about 78% nickel and 22% iron which is described in United States Patent 1,586,883 is suitable for this purpose. By reference to this patent it will be noted that this material has a much higher permeability and appreciably lower hysteresis loss than the best magnetic iron. If the strength of the permanent magnet changes, the calibration of the instrument will be but little affected since the permanent magnet flux is a factor in producing both driving and restoring torques. The ordinary return spring may be used, but the magnetic vane is simpler and less apt to get out of adjustment and is unaffected by temperaturechanges.

In vplace of metallic cups for bearings I may use the arrangement shown'in Fig. 3 where 28 represents a polished stone bearing face held in a metal housing 29. Above the stone 28 the housing contains a small amount of mercury 30 held in place by the washer 31. This Vmay be used for both the bottom and top bearings since the mercury will not work out even if the bearing is inverted. This makes a good electrical contact between the moving and stationary parts of the instrument-and should be used Where the instrument current is high as in ammeters.

The absence of spiral springs for current leads and return spring is highly advam tag'eous in miniature instruments of the character contemplatedbecause of the diiculty of manufacturing and adjusting line spiral springs.

Reliable instruments of the. form shown in Fig. 1 have been built in about the same proportions as representedwith an outside diameter of from 2 to '3 inches.

In accordance with the provisions of the patent statutes I have described the principle of operation of my invention, together with the apparatus which I now consider to represent the best embodiment thereof, but I desire to have it understood that the apparatus shown and described is only illustrative and that the invention may be carried out. by other means.

`What I claim as new and ydesire to secure by Letters Patent of the United States, is 1. In an electric measuring instrument, a. stationary field magnet provided with an air gap, a movable armature member in said air gap, and means for producing a zero restoring torque for said armature member comprising a magnetic vane movable with the armature member but mounted outside of the influence of its eld together-'with auxiliary pole pieces for diverting iiux from v said field magnet through said!y vane along lmeability ,andI lower hysteresis an/axis correspondin to the zero position of the vane, saidauxi iary pole pieces being .angularly adjustable withrespect to the axis of rotation of said'armature member to vary the zero setting thereof.

e 2. An electric measuring instrument comprising a permanent magnet provided with an air gap, a moving coil armature in said air gap, and means @for producing a z ero restorlng torque for said armature compris'.

ing `a magnetic vanev having higlher pert an iron movable with vsaid armature but mounte outside the inuence o its magnetic field,

and a pair of auxiliary pole pieces for diverting flux from said permanent magnet through said vane, said auxiliary pole pieces being separated from the permanent magnet by air gaps and adjustable about the axis of rotation of saiti vane for adjusting the zero position of said armature.

In witness whereof, I have lhereunto set of se t., 192s.

my hand this 24 da l ARO T. FAUS. 

